Thermoplastic resins exhibit excellent physical properties, such as low specific gravity, good moldability, and good impact resistance, as compared with glass or metal. Recently, with the trend of producing low cost, larger and lighter weight electronics, plastic products made of thermoplastic resins are quickly replacing existing glass or metal-based products, thereby broadening application ranges of the thermoplastic resins to fields from electronics to automobile components.
To impart flame retardancy to the thermoplastic resins, a method of blending antimony, halogen, phosphorus compounds, or compounds containing nitrogen as a flame retardant is widely known. Among such flame retardants, when a halogen flame retardant is used, there are drawbacks in that the halogen flame retardant causes corrosion of a processing apparatus due to corrosive gases generated therefrom during processes, and generates toxic gases such as dioxin, hydrogen halide gases and the like during burning. Thus, demand for resins free from halogen flame retardants is increasing.
The most general technique for imparting flame retardancy without use of the halogen flame retardants is use of phosphorus flame retardants. However, although phosphorus flame retardants are superior to the halogen flame retardants in terms of suppression of corrosive gases and toxic gases, phosphorus flame retardants exhibit lower flame retardancy than halogen flame retardants and have a drawback in that polymeric resins capable of acting as a flame retardant are limited to resins containing polycarbonate and polyphenylene ether. In addition, a higher amount of a phosphoric acid ester flame retardant causes deterioration in heat resistance of a resin composition. Further, the phosphoric acid ester flame retardant has problems in that a molded article suffers from a juicing phenomenon causing the phosphoric acid ester flame retardant to move to a surface of the molded article, or in that an injection-molded article suffers from black points and black lines on a surface thereof due to decomposition of the flame retardant, and deteriorates mechanical strength, such as impact strength, flexural strength, flexural modulus, and the like.
To resolve such problems, a method of introducing a flame retardant into a polymer chain has been proposed. For example, U.S. Pat. Nos. 3,725,509 and 4,014,836 disclose use of a copolymer of a halogen-containing unsaturated monomer and bis(hydrocarbyl)vinyl phosphonate as a flame retardant additive, and U.S. Pat. Nos. 4,444,969 and 4,571,418 disclose flame retardant copolymers prepared by copolymerization of an aromatic vinyl monomer, bis(hydrocarbyl)vinyl phosphonate, and an imide derivative of unsaturated anhydride. In addition, U.S. Pat. No. 4,035,571 discloses a flame retardant copolymer prepared by copolymerization of an unsaturated monomer, bis(hydrocarbyl)vinyl phosphonate, and acrylic or methacrylic acid.
However, such methods and copolymers have drawbacks in that a large amount of bis(hydrocarbyl)vinyl phosphonate must be introduced to secure sufficient flame retardancy, thereby extremely limiting use of the copolymers due to deterioration in heat resistance and mechanical properties.
Therefore, there is a need for a novel flame retardant thermoplastic copolymer, which can prevent deterioration in heat resistance caused by simple addition of a phosphorus flame retardant, and impart flame retardancy.